When NASA's Lunar Gateway space station begins operations later this decade, it won't orbit the Moon the way the International Space Station orbits Earth. Instead, it will trace a long, looping path through cislunar space called a Near-Rectilinear Halo Orbit , or NRHO. The orbit is unusual by design: at closest approach, the Gateway skims just 1,500 kilometers above the lunar surface, then swings out nearly 70,000 kilometers on its far pass, taking about 6.5 days to complete one loop. That dramatic shape isn't a compromise or an accident. The NRHO sits at a gravitational sweet spot in the Earth-Moon system that gives mission planners a combination of properties no other orbit provides: low fuel costs to maintain, continuous line-of-sight to Earth, frequent access to the lunar south pole, and relatively easy transfers to and from lunar orbit. Understanding why NASA chose this orbit means understanding the strange orbital mechanics of cislunar space. AI-generated image The five Lagrange points in the Earth-Moon system. The NRHO associated with L2 is Gateway's home orbit. Credit: Visualization The Three-Body Problem and Lagrange Points To understand the NRHO, you first need to understand the gravitational environment Gateway operates in. Any spacecraft in cislunar space isn't just pulled by Earth or just by the Moon. It feels both gravitational forces simultaneously, and the interaction between them creates a much more complex orbital landscape than the simple two-body physics that governs satellites in low Earth orbit. In 1772, mathematician Joseph-Louis Lagrange identified five special points in any two-body gravitational system where a third, much smaller object can remain in a stable or semi-stable position. These are the Lagrange points , labeled L1 through L5. In the Earth-Moon system, they sit at specific locations where the gravitational pull from both bodies, combined with the centrifugal force from the system's rotation, cancel out. L1 Between Earth and Moon, about 58,000 km from the lunar surface. Good for constant sun exposure, not ideal for lunar access. L2 Beyond the Moon from Earth, ~65,000 km from the lunar surface. Gateway's chosen Lagrange point. Strong line-of-sight to lunar south pole. L3 On the opposite side of Earth from the Moon. Unstable and impractical for any operational station. L4 60 degrees ahead of the Moon in its orbit. Naturally stable — objects there tend to stay. L5 60 degrees behind the Moon in its orbit. Also stable. Both L4 and L5 are far from the lunar surface and less useful for operations. Halo Orbits Three-dimensional loops around L1, L2, or L3. More practical than sitting exactly at a Lagrange point, which is unstable. L1, L2, and L3 are unstable equilibria. Place an object exactly there, and the slightest perturbation will send it drifting away. But orbits around these points, called halo orbits or Lissajous orbits, can be quasi-stable with periodic station-keeping burns. The spacecraft needs small thruster corrections every so often, but far less propellant than maintaining a conventional orbit at a similar altitude would require. Halo orbits around L2 are three-dimensional loops that circle the L2 point in a roughly circular or elongated path. The NRHO is a specific family of these halo orbits, distinguished by its high inclination and very high eccentricity, meaning the orbit's closest and farthest points differ by a factor of roughly 45. What Makes NRHO Different: The Geometry The name "near-rectilinear" refers to the orbit's shape in a Moon-centered reference frame. At low amplitudes, halo orbits near L2 look roughly circular. As you push toward higher amplitudes, the orbit elongates until, at the extreme, it looks almost like a straight line. The NRHO occupies this "near-rectilinear" regime: still technically a halo orbit, but pulled into a highly elongated, nearly polar path. 1,500 km Closest approach (perilune) above lunar surface 70,000 km Farthest point (apolune) from lunar surface 6.5 days Orbital period, one complete loop <10 m/s Annual delta-v for station-keeping 9:2 Synodic resonance with lunar orbit 730 m/s Delta-v from NRHO to low lunar polar orbit The orbit is polar, meaning it passes over the lunar north and south poles on each loop. This is critical for south pole access. The south pole is where water ice concentrations exist in permanently shadowed craters, and where the primary Artemis landing sites are located. A polar NRHO guarantees Gateway always has line-of-sight to the south pole region when it passes overhead. The 9:2 resonance number means Gateway completes exactly nine NRHO orbits for every two times the Moon orbits Earth. This resonance keeps the geometry predictable and repeat-able, which matters for coordinating mission operations. Astronauts departing Gateway to land on the surface need to plan their trajectory months in advance, and resonant orbits make those calculations far more tractable. Why Not a Low Lunar Orbit? A station in low lunar orbit would need thousands of meters per second of delta-v annually just to maintain altitude, because the Moon's irregular gravity (from mass concentrations called "mascons" left over from ancient asteroid impacts) perturbs low orbits continuously. The NRHO sidesteps this problem entirely. Its orbital mechanics are dominated by the three-body dynamics of the Earth-Moon system rather than the Moon's lumpy gravity, making it far cheaper to maintain. Getting There: Transfers to and from the NRHO One of the NRHO's most practical advantages is the set of trajectories that connect it to the rest of the cislunar system. Transfers between the NRHO and Earth, low lunar orbit, or the lunar surface are all manageable with modest propellant budgets, especially compared to the alternatives. AI-generated image Artist's concept of Orion approaching Gateway in the NRHO. The station's highly elongated orbit puts it sometimes very close to the Moon and sometimes very far away. Credit: AI visualization Earth to NRHO The Orion spacecraft's path from Earth to the NRHO uses a trajectory called a Distant Retrograde Orbit insertion or a direct translunar injection depending on the specific mission profile. NASA has studied multiple trajectory families. The current Artemis architecture uses Orion with a total transit time of about four to five days from launch to Gateway rendezvous. The key advantage: the NRHO's location near L2 makes it reachable from translunar injection with a single relatively modest insertion burn, in the range of 500 to 900 meters per second delta-v, depending on the specific trajectory chosen. NRHO to Lunar Surface Getting from Gateway to the lunar surface and back requires an intermediate step. Landers, including the SpaceX Starship Human Landing System for Artemis, descend from the NRHO by first burning into a low lunar orbit and then descending from there. The transfer from NRHO to low lunar polar orbit costs approximately 730 meters per second delta-v. From low lunar orbit, descent to the surface requires another 1,900 meters per second or so. The return trip reverses the process: ascent from the surface, orbit rendezvous, then transfer back to the NRHO for crew handoff to Orion. NRHO to Deeper Space Gateway's NRHO also sits in a useful location for future missions beyond the Moon. From L2, transfers to near-Earth asteroids and Mars-bound trajectories are more energetically accessible than from low Earth orbit. This is one of NASA's arguments for Gateway as infrastructure for deep space exploration, not just lunar surface access. Departing from the NRHO rather than Earth orbit saves propellant because the spacecraft is already partway up Earth's gravity well. CAPSTONE: Proving the Orbit Works Before committing Gateway to the NRHO, NASA wanted real flight data confirming the orbit behaves as orbital mechanics models predict. The CAPSTONE mission, short for Cislunar Autonomous Positioning System Technology Operations and Navigation Expe